Cooking process using hot black liquor

ABSTRACT

This invention refers to a method for producing pulp in an alkaline digesting process comprising the following steps: charging lignocellulosic material to a digester system; pre-treating said lignocellulosic material with an impregnation liquor and subsequently with a hotter liquor containing black liquor, simultaneously displacing spent liquor; heating and cooking said lignocellulosic material to produce cooked lignocellulosic material and cooking liquor; displacing a portion of said cooking liquor at cooking temperature from said digester to a first section by supplying a washing liquid; further displacing a second portion of displaced liquor from said digester system to at least a second section by supplying further washing liquid until the temperature of said second portion of displaced liquor drops to a temperature of at least about the boiling point of said cooking liquor at atmospheric pressure, said second portion of displaced liquor having a dry solids content substantially lower than the initial dry solids content of the cooking liquor and substantially lower than that of said first portion of displaced liquor. The method is characterized in that substantial parts of said second portion of displaced liquor is heated by the heat of the first portion of displaced liquor, whereby said first portion having a high solids content is prevented from being reused in a digester and said heated second portion is further used as hot liquor in the pre-treatment and heating of said lignocellulosic material.

FIELD OF INVENTION

[0001] The present invention concerns a method for producing pulp in an alkaline digesting process comprising the following steps:

[0002] charging lignocellulosic material to a digester system;

[0003] pre-treating said lignocellulosic material initially with an impregnation liquor and subsequently with at least a hotter liquor containing substantially black liquor, spent liquor being displaced from the digester during said treatment;

[0004] heating and cooking said lignocellulosic material at a cooking temperature producing cooked lignocellulosic material and cooking liquor, said cooking liquor being at said cooking temperature and having an initial dry solids content;

[0005] displacing a portion of said system cooking liquor at said cooking temperature from said digester to a first section by supplying a washing liquid to said digester in order to obtain a first portion of displaced cooking liquor having a first temperature and a first dry solids content;

[0006] further displacing a second portion of displaced liquor from said digester to at least a second section by supplying further washing liquid until the temperature of said second portion of displaced liquor drops to a temperature of at least about the boiling point of said cooking liquor at atmospheric pressure, resulting in a second portion of displaced liquor having a dry solids content substantially lower than said initial dry solids content of said cooking liquor and substantially lower than said first portion of displaced liquor.

BACKGROUND OF THE INVENTION

[0007] Common for displacement batch cooking of lignocellulosic material is that the free cooking liquor or so-called mother liquor at the end of cooking is displaced to recover heat and residual chemicals to be used in next batches. The drawback is that the re-circulated hot black liquor typically has high dry solids content and ionic strength. Further a build-up of dry solids in said liquor occurs between hot black liquor treatment and cooking. This occurs for example in the so called RDH, EnerBatch and SuperBatch displacement batch methods, where the hot cooking liquor or free liquor present in the digester at the end of cooking is re-used as hot black liquor in the next batches as hot liquor fill. Such systems are disclosed in e.g. U.S. Pat. No. 5,578,149 and FI patent 92224. In the next batch, the reuse of hot black liquor of high dry solids content will further enrich dry solids in the digester. The drawback is that high dry solids content in cooking impairs bleachability, at least for many hardwoods, and subsequent washing, because more dry solids bleed over to subsequent pulp washing. This may impair pulp quality and possibly increase alkali consumption in a cooking plant. The increased dry solids content in the beginning of washing has especially a negative impact on softwood pulping, because the higher dry solids content in the pulp suspension will cause low solubility of soap (extractives), when chips are defibrized during discharging the digester. This will result in high share of unsolubilized soap in the pulp suspension, which will impair dewatering and displacement in first stages of pulp washing. Thus, lower washing efficiency occurs and/or larger volumes of liquors have to be evaporated in the recovery plant or a higher number of wash equipments have to be used.

[0008] The same problem occurs in continuous digesters, e.g. a method described in WO 98/35090, using spent cooking liquor, normally the so-called mother liquor, in a hot black liquor treatment prior to the cooking stage. The dry solids content is enriched in the recirculation between pre-treatment and cooking step, which thereby will increase the load on washing.

SUMMARY OF THE INVENTION

[0009] The object of the invention is to increase washing efficiency in the digester and to decrease the dry solids content of the cooking liquor in the digester when using hot black liquor treatments prior to cooking. This is achieved with a method according to the preamble, characterized by the features that substantial parts of said second portion of displaced liquor is heated by the heat of the first portion of displaced liquor, whereby said first portion having a high solids content is prevented from being reused in the digester, and said heated second portion is further used as hot liquor in the pre-treatment and heating of said lignocellulosic material. Due to these features the dry solids content in the cooking liquor, and subsequent process stages, such as washing and bleaching will decrease significantly. This will result in easier washing and higher washing efficiency in the whole plant, in improved operation, improved pulp quality and bleachability without essentially affecting production capability or steam economy.

[0010] Further features of the invention will appear from the enclosed independent claims.

[0011] The invention will be described in more detail in the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows a prior art embodiment according to a so called twin tank process, i.e. including two hot black liquor tanks;

[0013]FIG. 2 shows another prior art embodiment provided for handling calcium-rich raw materials;

[0014]FIG. 3 shows an embodiment according to the present invention also including the process for handling calcium-rich raw materials;

[0015]FIG. 4 shows an embodiment according to the invention comprising only one HBL tank divided into two sections, thus different from the system of FIG. 3, and

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] For clarity in the different figures, functionally corresponding tanks carry the same number, although the process varies. Valves, instruments and control loops are not shown, but it is obvious to the person skilled in the art that these may be provided at any location to enable any desired flow path. In the figures, the required tanks are shown at the top, while the sequence of steps carried out in the digester system is diagrammatically shown at the bottom with corresponding liquor flows indicated.

[0017] In an alkaline cooking process lignocellulosic material, such as wood chips is charged into the digester system. The lignocellulosic material is pre-treated with an impregnation liquor A, having a temperature of below 120° C., preferably below the boiling point of said impregnation liquor from an atmospheric tank. In a process where calcium-rich raw materials are handled, e.g. in a process as shown in FIGS. 3 and 4 according to the invention and in the prior art process shown in FIG. 2 said impregnation liquor A is fed from a wash filtrate tank 6. A smaller amount of white liquor can be added to said impregnation liquor A in order to adjust the alkali content. In processes shown in FIG. 1 according to the prior art and in FIG. 5 according to the invention, the impregnation liquor A is fed from an impregnation liquor tank 5, preferably atmospheric, receiving cooled black liquor from a hot black liquor tank 2. Soap is removed from wash filtrate tank 6, impregnation liquor tank 5 and black liquor tank 4 and the soap is further conducted to recovery.

[0018] Subsequently the lignocellulosic material is pre-treated with at least a hotter liquor B, having a temperature of about 120 to 180° C., and containing substantially black liquor from hot black liquor tank 1. The pre-treatment is preferably carried out for a time and temperature corresponding to an H-factor of below 300.

[0019] Before cooking hot white liquor C is added with some hot black liquor from tank 1 to the digester system. Cold spent liquors A1 and A2 from the impregnation liquor and hot liquor treatment are displaced from digester during these two pre-treatment stages, which spent liquors are fed to black liquor tank 4 at point AB. Tank 4 is preferably atmospheric. The displaced liquors above about the atmospheric boiling point are conducted to pressurized tanks.

[0020] The lignocellulosic material is then heated and cooked at a cooking temperature of 140 to 185° C., preferably 150 to 180° C., whereby cooked lignocellulosic material and cooking liquor is produced. The cooking liquor is at said cooking temperature and it has an initial dry solids content.

[0021] After completed cooking a first portion D1 of the cooking liquor at said cooking temperature is displaced from the digester to a first section, i.e. to the hot black liquor tank 2 at point D, according to the invention, as shown in FIGS. 3-4, by supplying a washing liquor E from the wash filtrate tank 6, in the process shown in FIG. 5. In the processes shown in FIGS. 3 and 4 according to the invention, the displacement is first carried out with a liquor E from the black liquor tank 4.

[0022] Further displacement is carried out by supplying washing liquor F from the wash filtrate tank 6 to the digester whereby a second portion of displaced liquor B1 is displaced to a second section, i.e. to black liquor tank 1, at point B. The displacement is carried out until the temperature of said second portion B1 of displaced liquor drops to a temperature of at least about the boiling point of said cooking liquor at atmospheric pressure. Said second portion B1 of displaced liquor will have a dry solids content substantially lower than said initial dry solids content of the cooking liquor and substantially lower than said first portion D1 of displaced liquor.

[0023] According to the present invention substantial parts of the second portion B1 of the discharged liquor in the hot black liquor tank 1 is heated by the heat of the first portion D1 of displaced liquor in the black liquor tank 2. Said second portion B1 of displaced liquor is then used as hot liquor B in the pre-treatment and heating of lignocellulosic material charged to the digester. The temperature of the hot liquor B is preferably adjusted with steam before entering the digester. Fresh liquor, i.e. white liquor may be added to the pre-treatment stage and/or to the heating/cooking stage in order to adjust the alkalinity of the liquor in the digester.

[0024] The dry solids content and the temperature of displaced liquor B1, D1 are monitored and the received results are utilized in controlling the process.

[0025] On the other hand, the prior art processes shown in FIGS. 1 and 2, displaces from the digester after completed cooking, a first portion B1 of cooking liquor at said cooking temperature to a first section, i.e. to the hot black liquor tank 1 at point B, by supplying a washing liquor E from the wash filtrate tank 6, in the process shown in FIG. 1. In the prior art process shown in FIG. 2, the displacement is first carried out with a liquor E from the black liquor tank 4.

[0026] According to prior art, further displacement is carried out by supplying more washing liquor E or F from the wash filtrate tank 6 to the digester, whereby a second portion of displaced liquor D1 is displaced to a second section, i.e. to black liquor tank 2, at point D. The displacement is carried out until the temperature of said second portion D1 of displaced liquor drops to a temperature of at least about the boiling point of said cooking liquor at atmospheric pressure.

[0027] According to prior art, said first portion B1 of displaced liquor will have a dry solids content close to said initial dry solids content of cooking liquor. Said second portion D1 will have a dry solids content substantially lower than said initial dry solids content and substantially lower than said first portion D1 of displaced liquor.

[0028] Thus it is obvious that in the pre-treatment stages of the prior art processes a hot black liquor is used containing substantial amounts of dry-solids causing problems in subsequent process stages, as mentioned previously.

[0029] According to the present invention displacement of cooking liquor D1 from the digester to black liquor tank 2 is preferably continued until the dry solids content of said displaced cooking liquor D1 drops below said initial dry solids content of cooking liquor, resulting in a first portion of displaced liquor having a temperature of at least over about 85% of the cooking temperature value in Celsius degrees and a dry solids content substantially corresponding to at least over about 85% of the dry solids content of said cooking liquor.

[0030] During the cooking stage, indicated by “Temperature adjustment” and “Cooking time” in the FIGS. 3 to 4 some liquor B2 and/or D2 will be displaced due to charge of further white liquor C into the digester for adjusting the alkalinity of the cooking liquor. By monitoring the dry solids content of said displaced liquor B2, D2 it will be fed to the hot black liquor tank 2, if the monitored dry solids content of the displaced liquor D2 is substantially at the same level as the initial dry solids content of the cooking liquor, and to the hot black liquor tank 1, if the monitored dry solids content is at a lower level.

[0031] According to the invention the first portion of displaced liquor D1, D2, having a high dry solids content, is prevented from being reused in the digester. Instead the displaced liquor D in the hot liquor tank 2, having a high dry solids content, will be conducted through heat recovery equipments to a chemical recovery plant, i.e. an evaporation plant. In a first liquid/liquid heat exchanger said first portion of displaced liquor D is used to heat the second portion of displaced liquor B1, B2, i.e. B fed to the hot black liquor tank 1. In a further heat exchanger the liquor D having high dry solids content is used to heat white liquor from recovery of cooking chemicals. Further said liquor D is used to heat water.

[0032] The temperature of the second portion of displaced liquor B, which has preliminary been heated by the first portion of displaced liquor D, will still be adjusted by steam before said liquor B is introduced into the digester. Further the alkalinity of said second portion of displaced liquor, i.e. liquor B is adjusted by fresh alkali, i.e. white liquor C, before said liquor B is introduced into the digester. Said adjustment of alkalinity is preferably carried out in a line from the hot black liquid tank 1 to the digester.

[0033] In the embodiment shown in FIG. 4 the gas space of the first section, i.e. section 2 of the hot black liquor tank communicates with the second section, i.e. section 1 of said hot black liquor tank. Due to this released steam from said first section 2 heats the liquor in said second section 1 and simultaneously the liquor B will be diluted while the dry solids content of the liquor D in said first section 2 will be increased, decreasing energy consumption in a subsequent evaporation plant. Sulphur compounds released from liquor D in the first section 2 will absorb into the liquor B in the second section 1 increasing the alkalinity of said liquor B.

[0034] Liquor B3 displaced from the digester during the pre-treatment is led to the second section, i.e. hot liquor tank 1 as soon as the temperature of said liquor B3 is above about 100° C. and the dry solids content of said liquor B3 is at about the same level as that of the second portion of displaced liquor B and/or the dry solids content of said liquor B3 is below that of the first portion of displaced liquor D.

[0035] According to another embodiment of the invention said second section 1 is divided into a plurality of subsections (not shown) containing liquors of various dry solids content. Substantial parts of the liquors in said sub-sections of second section are used as hot liquor B in the pre-treatment, such that the order of addition to the digester system is from highest to lowest dry solids content.

[0036] The second section 1 and its subsections are preferably provided with mixing or other similar arrangement resulting in uniform composition and properties of the liquor B.

[0037] The dry solids content of the wash filtrate F is lower than that of the initial cooking liquor, said first portion of displaced liquor D and said second portion of displaced liquor B. The idea is that the dry solids content of the liquors added to the digester during a digesting sequence is gradually lowered in each subsequent charge of liquor to the digester in order to keep the total dry solids content in the digester substantially constant, and especially to prevent build up of dry solids in the digester system. The temperature of the washing filtrate F is below the boiling point of said washing liquid, preferably about 85 to 50° C.

[0038] In the enclosed table 1 the results from simulated digesting processes corresponding to the prior art process according to FIG. 1 (Examples 1 and 2) and digesting processes according to the present invention, corresponding to the process shown in FIG. 3 (Examples 3 to 7).

[0039] The Examples show how the new process according to the invention will decrease the dry solids content in the liquors using alkaline cooking with a yield of 46% and oxygen delignification with a yield of 97%, as well as during washing with dilution factor (DF) with water and different washing efficiency values according to the E10-factor. At the end of the cook the dry solids content in the liquor is decreased from 23.4% to 19.1-18.8% when a method according to the invention is used. In hot black liquor B the dry solids content will decrease from 21.2% to 14.1-13.7% and in the wash filtrate from 12.7% to 10.9-10.3. The dry solids content of the impregnation liquor will decrease from 17.3% to 10.9-10.3%. Thus, the dry solids content of all liquors will decrease considerably by using a method according to the present invention. This can be achieved without significantly affecting the steam consumption.

[0040] The advantage of the invention is, that the pulp washing efficiency is improved considerably because the amount of dissolved substances in the pulp leaving the digester is considerably lower. This is clearly indicated by the fact that the dry solids content of the wash filtrate is considerably lower. This decrease of dry solids content leads for instance to considerably easier soaking of released material from the cooked ships and the fibres. For instance in cooking extractive containing wood this enables a considerably increased solubility of difficultly soluble extractive matters. This decrease in dry solids content makes the dewatering and washing of the pulp considerably easier, and the amount of required anti foaming agent to be used will decrease remarkably. The share of unwished unoxidized material from the cooking in the inlet to the oxygen delignification stage is also considerably smaller, especially using a pulp washing equipment with lower E10-values, cf. for instance Example 1 and Example 3. Further it may be assumed that the average E10-value will be improved by a specific pulp washing equipment in using a method according to the invention. This will further decrease wash loss of unoxzidized COD in the inlet to the oxygen delignificaton stage, cf. for instance Example 1 (prior art) and Example 5. It can be used to decrease the dilution factor DF in the pulp wash, which increases the dry solids content of liquor from the digesting room to be evaporated, which will decrease energy consumption in black liquor evaporation and increase the capacity of the evaporation plant, cf. Example 2 and Example 7.

[0041] It may also be assumed that the decreased dry solids content of the spent liquors in the cooking will improve and simplify bleaching of the cooked pulp, which has been experienced especially in pilot plant cooking of hardwood. This decreased dry solids content may also be used as a means to decrease the amount of fresh white liquor in the digesting room.

[0042] Example 9 is an example of a method without using liquid/liquid heat exchangers. As will be seen from the table the steam consumption for such a process is considerably higher. TABLE 1 Example 1 2 3 4 5 6 7 9 Steam consumption in cooking LP (kg/adt) 210 210 210 210 210 210 210 210 MP (kg/adt) 664 665 676 676 676 676 683 1134 Dry solids contents of cooking liquors Impregnation liquor (A) (%) 17.3 17.3 10.8 10.9 10.3 10.3 11.7 10.9 Hot black liquor (B) (%) 21.2 21.2 14.1 14.1 13.7 13.7 14.8 14.2 End-of-cook (%) 23.4 23.4 19.1 19.1 18.8 18.8 19.4 19.2 Wash filtrate (F) (%) 12.7 12.7 10.7 10.9 10.3 10.3 11.7 10.9 Evaporation black liquor (%) 16.5 16.5 16.5 16.5 16.5 16.5 17.2 16.6 Washing E10-values (washing prior 11.5 14.9 11.5 14.9 12.6 16.0 16.4 15.0 oxygen delignification) DF (on last washer) 2.5 2.5 2.5 2.5 2.5 2.5 2 2.5 COD wash loss (total/ 155/19 121/6.3 144/15.6 118/5.1 132/11.1 114/3.6 139/6.1 118/5.1 unoxidized in pulp to oxygen (kg/adt) delignification) Cooking yield (%) 46 O2 delignification yield (%) 97 O2 inlet consistency (%) 11 

What we claim:
 1. A method for producing pulp in an alkaline digesting process comprising the following steps: charging lignocellulosic material to a digester system; pre-treating said lignocellulosic material initially with an impregnation liquor and subsequently with at least a hotter liquor containing substantially black liquor, spent liquor being displaced from the digester during said treatment; heating and cooking said lignocellulosic material at a cooking temperature producing cooked lignocellulosic material and cooking liquor, said cooking liquor being at said cooking temperature and having an initial dry solids content; displacing a portion of said cooking liquor at said cooking temperature from said digester to a first section by supplying a washing liquid to said digester in order to obtain a first portion of displaced cooking liquor having a first temperature and a first dry solids content; further displacing a second portion of displaced liquor from said digester system to at least a second section by supplying further washing liquid until the temperature of said second portion of displaced liquor drops to a temperature of at least about the boiling point of said cooking liquor at atmospheric pressure, resulting in a second portion of displaced liquor having a dry solids content substantially lower than said initial dry solids content of said cooking liquor and substantially lower than said first portion of displaced liquor, characterized in that substantial parts of said second portion of displaced liquor is heated by the heat of the first portion of displaced liquor, whereby said first portion having a high solids content is prevented from being reused in the digester, and said heated second portion is further used as hot liquor in the pre-treatment and heating of said lignocellulosic material.
 2. Method according to claim 1, characterized in that the dry solids content and temperature of displaced liquor are monitored.
 3. Method according to claim 2, characterized in that displacement of cooking liquor to the first section is continued until the dry solids content of the displaced cooking liquor drops below said initial dry solids content of cooking liquor resulting in a first portion of displaced liquor having a temperature of at least over about 85% of the cooking temperature value in Celsius degrees and a dry solids content substantially corresponding to at least over about 85% of the dry solids content of said cooking liquor.
 4. Method according to claim 3, characterized in that liquors displaced during the cooking stage is conducted to first or second section dependent on monitored dry solids content of said liquors.
 5. Method according to claim 1, characterized in that the first portion of displaced liquor having a high dry solids content is conducted through heat recovery equipments to a chemical recovery plant, i.e. an evaporation plant.
 6. Method according to claim 1, characterized in that the second portion of displaced liquor is heated by the first portion of displaced liquor in a liquid-liquid heat exchanger before introduction into the digester.
 7. Method according to claim 5, characterized in that heat of first portion of displaced liquor is used in heating white liquor.
 8. Method according to claim 6, characterized in that the temperature of the second portion of displaced liquor is adjusted by steam before it is introduced into the digester.
 9. Method according to claim 8, characterized in that alkalinity of the second portion of displaced liquor is adjusted by fresh alkali before said liquor is introduced into the digester.
 10. Method according to claim 9, characterized in that the alkalinity of the second portion of displaced liquor is adjusted in a line from said second section to the digester.
 11. Method according to claim 9, characterized in that fresh liquor, i.e. white liquor is added to the pre-treatment stage and/or to the heating/-cooking stage in order to adjust the alkalinity of the liquor in the digester.
 12. Method according to claim 1, characterized in that gas space of the first section communicates with the second section, whereby released steam from said first section heats the liquor in said second section simultaneously diluting said liquor and increasing the dry solids content of the liquor in said first section.
 13. Method according to claim 12, characterized in that sulphur compounds released from the liquor in said first section absorb into the liquor in said second section.
 14. Method according to claim 1, characterized in that liquor displaced from the digester during the pre-treatment is led to the second section, as soon as the temperature of said liquor is above about 100° C. and the dry solids content of said liquor is at about the same level as that of the second portion of displaced liquor and/or the dry solids content of said liquor is below that of the first portion of displaced liquor.
 15. Method according to claim 1, characterized in that said second section is divided into a plurality of subsections containing liquors of various dry solids content.
 16. Method according to claim 15, characterized in that substantial parts of the liquors of the subsections of second section are used as hot liquor in the pre-treatment such that the order of addition to the digester system is from highest to lowest dry solids content.
 17. Method according to any of the preceding claims, characterized in that said second section and its subsections is/are provided with mixing.
 18. Method according to any of the preceding claims, characterized in that the dry solids content of the wash liquor is lower than that of said initial cooking liquor, said first portion of displaced liquor and said second portion of displaced liquor.
 19. Method according to claim 1, characterized in that heat from first portion of displaced liquor is used to heat the lignocellulosic material during charging to the digester.
 20. Method according to claim 19, characterized in that said heating is carried out by flashing, using flash steam.
 21. Method according to claim 1, characterized in that the temperature of the impregnation liquor is below about 120° C., preferably below the boiling point of said impregnation liquor
 22. Method according to claim 1, characterized in that the temperature of the hotter liquor is about 120 to 180° C.
 23. Method according to claim 1, charact rized in that pre-treatment of the lignocellulosic material is carried out for a time and temperature corresponding to an H-factor of below
 300. 24. Method according to claim 1, characterized in that the temperature of the washing liquid is below the boiling point of said washing liquid, preferably about 85 to 50° C.
 25. Method according to claim 1, characterized in that said cooking temperature is 140 to 185° C.
 26. Method according to claim 1, characterized in that the invention is applicable both on batch and continuous digesting processes. 